1. Field of the Invention
The present invention relates to the field of refrigeration.
2. Background of Art Related to the Invention
For many decades, domestic refrigerators have included a freezer section and a fresh food section. The fresh food section is maintained at a significantly higher temperature than the freezer section. While the basic laws of thermodynamics provide empirical evidence that it is increasingly more difficult to cool (i.e., remove heat from) an item as its temperature decreases, domestic refrigerators typically have been designed with more consideration focused on cost than thermodynamics. For example, many domestic refrigerators use a one-stage refrigeration system including a single evaporator located in the freezer section. Since the total heat load dissipation is through this single evaporator, this one-stage refrigeration system possesses less than optimal energy efficiency.
Recently, in order to increase system efficiency, some refrigerators have been constructed with two separate refrigeration systems; namely, one refrigeration system is responsible for cooling the freezer section while the other refrigeration system is responsible for cooling the fresh food section. Consequently, this dual refrigeration system includes repetitive condensing units, each featuring a compressor and a condenser. This repetition of equipment increases the cost and size of the refrigerator. Also, these repetitive condensing units produce a greater amount of noise.
Another example involves yacht refrigerators which have been implemented with refrigeration systems having valves to sequentially, but not simultaneously, connect a single, high-capacity condensing unit to multiple evaporators operating at differing temperatures. The refrigeration system may use thermal energy storage (TES) material to provide stable temperatures during the period between evaporator operations.
Preferably, TES material is an aqueous solution such as a salt solution having water and sodium chloride (NaCl). This composition provides high heat storage capacity, emits a large amount of heat isothermally upon changing phase from a liquid to a solid, is-non-toxic and can be produced for a low cost. Unfortunately, this TES material is highly corrosive to most metals, tends to expand when frozen which would damage the thin wall of the heat exchanger and tends to freeze first on the heat exchange surfaces which would hamper further heat transfer. This requires the TES material to be separated from the thin-walled metal tubing of the heat exchanger. One technique of separation involves encapsulating TES material into separate expandable capsules as described in U.S. Pat. No. 5,239,839 by the named inventor.
Additionally, the use of TES material adversely affects the efficiency of conventional defrosting cycles. The reason is that conventional defrost methods, if implemented, would require the entire TES material to melt before actual defrosting could begin.
U.S. Pat. Nos. 4,712,387 and 4,756,164 by the named inventor describe a heat pipe based method for efficiently transferring heat into and out of TES material and a method for thermally de-coupling the TES material from the cooled space to enable simple and efficient defrosting of the evaporator. These methods fail to provide any suggestion of the multi-stage refrigeration system and/or control protocol used to control this refrigeration system.
PCT International Patent Application No. PCT/US97/20151 describes a cost-effective evaporation unit and an energy efficient control protocol to maintain steady temperatures for each section of a refrigeration unit. An additional element of that disclosure is the use and design of a simple sensor for determining the frozen fraction of a TES module in order to control on-and-off cycling of the compressor for temperature stabilization.
The present invention comprises a method using controlled forced-convection to couple (or de-couple) a refrigerant-evaporator thermally to a refrigerated compartment. When thermally coupled to its compartment, an evaporator can provide efficient cooling to the compartment. The ability to de-couple the evaporator from its compartment enables refrigerant flow through the evaporator at significantly different temperatures than its compartment, but without significant heat transfer. This ability enables using a single refrigeration system (with no valves) to remove heat (sequentially) at two or more different temperatures from two or more refrigerated compartments. This capability enables achieving the maximum energy efficiency for the heat removal process from a multi-temperature system (e.g., a domestic refrigerator/freezer appliance) with a single refrigeration system.
The present invention also comprises a method to use heat from a fresh food compartment (or other compartment maintained above 0 C) of a two or more temperature refrigeration appliance to defrost a freezer evaporator automatically without using a controller or heater. The benefits of this innovation come from use of this innovation in conjunction with one of the frost-free methods also disclosed herein.